A pair of specimens with statistically significant difference in IASCC (irradiation-assisted stress corrosion cracking) response that cannot be explained by existing crack growth rate (CGR) models were characterized to identify key microstructural variables responsible for variability in IASCC response. Metallographic samples were prepared from the CGR specimens (Types 316 and 316Ti at 25 dpa). Light optical and scanning electron microscopy (SEM) were used to document the general microstructure of the materials, and microhardness measurements were performed to document variations within and between specimens. The ferrite contents were measured with a dedicated instrument. SEM/electron backscatter diffraction (EBSD) was used to determine differences in grain size, grain boundary misorientation, texture, ferrite characteristics, Schmid and Taylor factors, and the number of geometrically necessary dislocations. Elemental compositions of austenite grain boundaries and austenite/ferrite phase boundaries were analyzed by atom probe tomography (APT). APT was also used to determine, characterize, and quantify the austenite as well as the ferrite with respect to the presence of radiation-induced precipitation and clustering. The present study shows that there is not a single factor or microstructural variable that alone can explain the difference in CGR. Consequently, several factors or microstructural variables must have acted in combination to result in the difference in CGR. The paper discusses the differences observed and their implications for IASCC.